When a person becomes middle aged, the accommodation power of the ocular lens becomes weak, so that it becomes difficult for the person to adjust the focus sharply in near vision though the person may see well in far vision. This is an unavoidable phenomenon caused by aging, the phenomenon being called prebyopia. A traditional measure to counter prebyopia is to use a multifocal lens having two or more different visual field portions corresponding to different visual distances. Of the different visual field portions, an auxiliary visual field portion is called "a segment". A great deal of development effort has been exerted for a long time into a segment constituted by an unifocal area having low astigmatism and moderate addition.
In the multifocal lens, however, there is no freedom to apply an ideal optical design thereon, because of the narrowness of the progressive focal area and the monotonousness of the progressive distribution. Accordingly, use of the technique in practice is attended with difficulties. Those multiple lenses belong to a technique having means different from those of the present invention and in each of those multiple focal lenses no progressive focal area is provided in its segment portion. Explanation will next be made as to comparison between the prior art Japanese Patent Unexamined Publication No. earlier mentioned and the present invention. 1. [Differences in Subject Matter]: The prior art is directed to a plastic lens based on the manufacturing method disclosed or taught therein, while the present invention is directed to general lenses including an inorganic glass lens and also to a bonded lens. 2. [Superiority]: In the prior art, progressive deformation is provided to a mold having a segment previously formed therein so that it is impossible to provide a correct progressive portion only to the segment, which results in that, since this is a conventionally existing progressive focal lens, the progressive deformation causes astigmatism and distortion to be left even in the main lens; whereas, in the present invention, such astigmatism and distortion are not present as a matter of course, since the entire main lens corresponds to a unifocal area with no progressive portion. Further, the present invention is superior to the prior art in that a freely designed progressive distribution can be provided only to the segment so that, even in the case of a sharply progressive design, this can affect only a step in the boundary of the segment and cannot affect the main lens at all. 3. [Uniqueness]: For the purpose of forming the shape of a segment, the prior art inevitably requires a refracting power difference of at least 0.5 diopter in the upper portion of the segment. According to the present invention, on the other hand, the shape of the segment can be set from the initial stage so that the refracting power can be set equal to that of the main lens, whereby stepless progression can be uniquely attained 4. [Differences in Object]: The prior art is different in object from the present invention in that the object of the prior art is to weaken the quantity of addition of the provided segment corresponding to the segment for the purpose of reducing the lateral astigmatism and distortion caused by the progression, while the object of the present invention is to realize the quantity of strong addition conversely with use of the progressive focal segment. 5. [Advanceability, Difficulty]: It is impossible to realize the provision of the progressive portion only to the segment or the provision of prism thereto without any restrictions, which forms a feature of the present invention, according to the manufacturing method disclosed or taught in the prior art. Such a feature can be realized only according to the high-degree unique manufacturing method of the present invention to be detailed later.
According to the present invention, it is intended that, changing conception apart from readly-made ideas, a main lens portion is left as it is as a unifocal area, and a segment having a limited size is used, while conversely utilizing the feature that the segment is small, to thereby extremely enlarge the optical capacity by application of a progressive focal function to the segment.